Microalgae cultures are an alternative, renewable source of oil. Research and development efforts in recent years have been directed to optimizing both the microalgae cultures themselves, and the systems and methods used for farming and processing them, with the goal of achieving large-scale, commercial deployment of microalgae oil production. Most algae farms select a microalgae species of desirable robustness, growth rate, and oil yield, and grow an algae monoculture in open ponds, closed ponds, or bioreactors. Each of these approaches has advantages and drawbacks.
Open ponds can be categorized as either natural waters (lakes, lagoons, ponds) or artificial ponds or containers. Most systems utilize large shallow ponds, tanks, circular ponds, or raceway ponds. Open ponds are typically easier to construct and operate than most closed systems, and thus provide significant capital and operation cost advantages. However, they have a large footprint (in particular, compared with photobioreactors), and their biomass productivity and efficiency are often limited by uneven light distribution in the pond and poor light utilization by the cells, evaporative losses, diffusion of CO2 to the atmosphere, and low mass transfer rates due to insufficient stirring mechanisms. Further, contamination by predators and other fast-growing heterotrophs have restricted the commercial production of microalgae in open culture systems to only those organisms that can withstand such intruders. In addition, open ponds allow little or no control over the temperature and other environmental conditions in and around the pond. As a result, the water temperature at which the production levels are maximized can be difficult to maintain, and weather conditions can stunt algae growth. The geographic regions deemed most suitable for such algae culture systems have been warm, tropical or dry climate zones (e.g., Israel, Brazil, Arizona, Hawaii).
Closed ponds combine open ponds with a translucent cover (e.g., made of plexiglass), which forms a greenhouse above the pond and facilitates greater control over the environment. This can eliminate many of the problems associated with an open system. It may allow the species that are being grown to stay dominant, extend the growing season, and facilitate optimal year-round production if the greenhouse is temperature-controlled. Further, the amount of carbon dioxide may be increased in these quasi-closed systems, which, in turn, increases the algae growth rate. On the downside, closed pond systems cost more than the open ponds, which usually results in a smaller system. Closed ponds are commonly used for commercial spirulina cultivation.
Algae can also be grown in a photobioreactor (PBR), e.g., a closed bioreactor which facilitates exposure of the microorganisms contained therein to light, for example, by being translucent or by incorporating an artificial light source. PBRs are typically more complex than open or closed ponds. Like closed ponds, PBRs allow the cultured species to stay dominant and extend the growing season slightly if unheated and to a full year if heated, but generally require significant cooling energy, especially in warm climates. Because PBRs are closed, all essential nutrients need to be artificially introduced into the system to allow algae growth. A PBR may be operated in “batch mode.” Alternatively, a continuous stream of sterilized water, containing nutrients, air, and carbon dioxide, may be introduced, and algae may be harvested as they grow and cause excess culture to overflow. Once a continuous PBR is successfully started, it can continue to operate for long periods. However, if sufficient care is not taken, continuous bioreactors can easily collapse. An advantage of continuous PBRs that harvest algae are generally in the “log phase,” containing higher nutrient content than old “senescent” algae. The maximum productivity for a bioreactor occurs when the “exchange rate” (i.e., the time to exchange one volume of liquid) is equal to the “doubling time” (e.g., in mass or volume) of the algae. To increase the commercial viability of oil production with microalgae, it is desirable to increase the oil production and overall efficiency of microalgae culture systems and facilitate microalgae growth under a wider range of environmental conditions.